The present invention relates to optical parametric oscillators which are optical devices pumped by a coherent light source.
Optical parametric amplification (OPA) is a nonlinear optical process whereby light at one wavelength, the pump wavelength, is used to generate light at two other (longer) wavelengths in a material having a non-vanishing nonlinear susceptibility. When such a pumped material is placed in an optical cavity, an optical parametric oscillator (OPO) results. The relatively weak nonlinearities of optical materials made OPA and OPO impractical until the advent of the laser, which provided an intense source of coherent light.
A schematic diagram of a prior art OPO appears in FIG. 1. The pump 10 provides a source of intense coherent radiation in the form of the pump wave 14. A suitable nonlinear optical material 13 is placed in the optical cavity formed by mirrors 11 and 12. Mirror 11 is essentially transparent to pump wave 14, thereby providing a pump source to nonlinear optical material 13. Mirror 12 is partially transparent to the signal wave 16, which along with the idler wave 15 is generated by nonlinear interaction of pump wave 14 with nonlinear optical material 13. For simplicity, FIG. 1 shows all three waves propagating along a phasematch or quasi-phasematch direction within nonlinear optical material 13, a situation known as collinear phase matching. More generally, collinearity of the three waves is not required for OPO function.
An average photon from signal wave 16 makes multiple passes through nonlinear optical medium 13 before escaping from the optical cavity through mirror 12. Such apparatus can provide reasonably efficient (10-20%) conversion of pump photons into signal photons.
Like excited optical laser media, OPA involves optical gain and amplification of light. In laser media, however, there is no fundamental relationship between the coherence or lack thereof of the excitation energy and the laser radiation. In contrast, in OPA the pump source must be coherent light, and the output energy is coherently coupled and phase-locked to the laser pump.
In a suitable nonlinear material, usually a birefringent crystal with a nonvanishing second order nonlinear susceptibility, optical gain is established at two wavelengths, conventionally referred to as the signal and idler wavelengths. The sum of the energies of a signal photon and an idler photon are equal to the energy of a pump photon. There is no fundamental physical distinction between the idler wave and the signal wave. For the present application, the signal wavelength is the desired output wavelength of the optical parametric oscillator.
To obtain a useful device, it is necessary to be able to choose a specific signal wavelength. This is made possible within the nonlinear material itself, as useful gain appears only when the pump wave, the signal wave, and the idler wave can propagate and stay in phase with each other.
This in-phase condition is difficult to establish. Optical materials generally exhibit a property called dispersion, in which the refractive index varies with wavelength. Normally, shorter wavelength light propagates more slowly than do longer wavelengths. Consequently, as waves with different frequencies propagate they rapidly move in and out of phase with each other. The resulting interference prevents the signal wave from experiencing significant optical gain.
However, as is known in the art, in some birefringent materials, for particular temperatures and propagation directions, the difference in refractive index between ordinary and extraordinary waves can be used to offset the effects of dispersion at a desired signal wavelength. This phase-matched condition allows phase coherence to be maintained as the beams propagate, resulting in growth of the signal and idler waves.
Another technique for obtaining optical gain in the active medium of an optical parametric oscillator is to periodically change the relative phase of the pump, signal, and idler waves in the medium, thereby preventing the phase difference between these waves from becoming large enough to reduce the optical gain of the nonlinear medium to a point where the oscillator will not function. This approach, called quasi-phase-matching, is typically accomplished by changing crystal properties (e.g., ferroelectric polarization direction) as a function of propagation distance through the crystal. This technique has been used in LiNbO.sub.3 and KTiOPO.sub.4 (KTP). Unless otherwise differentiated, use of the term "phase-matching" and related terms such as phasematched will include the equivalent condition accomplished using quasi-phase matching.
Suitable nonlinear optical media include KTiOPO.sub.4 (KTP) and its isomorphs, KH.sub.2 PO.sub.4 (KDP) and its isomorphs, LiNbO.sub.3 and its isomorphs, potassium pentaborate tetrahydrate (KB5) and its isomorphs, lithium formate (LFM) and its isomorphs, Ca.sub.4 GdO(BO.sub.3).sub.3 and its isomorphs, Se, Te, III-V semiconductors, II-VI semiconductors, semiconductor quantum-well materials, HgS (cinnabar), quartz, Ag.sub.3 AsS.sub.3 (proustite) and its isomorphs, LiB.sub.3 O.sub.5, Li.sub.2 B.sub.4 O.sub.7, KBe.sub.2 BO.sub.3 F.sub.2, .beta.-BaB.sub.2 O.sub.4, AgGaS.sub.2, .alpha.-HIO.sub.3, BeSO.sub.4.4H.sub.2 O, HgGa.sub.2 S.sub.4, ZnGeP.sub.2 (ZGP), barium-sodium niobate, Sr.sub.x B.sub.1-x Nb.sub.2 O.sub.6 (SBN), PbB.sub.4 O.sub.7, CdHg(SCN).sub.4, Gd.sub.2 (MoO.sub.4).sub.3, Tl.sub.3 AsSe.sub.3 and its isomorphs, CsLiB.sub.6 O.sub.10, urea, cesium dihydroarsenate, and L-arginine phosphate. The instant invention may be implemented using any of the above materials, and any other nonlinear optical materials having suitable properties which may presently exist or be introduced in the future.
A prior art technique to reduce the amount of pump energy required to obtain efficient conversion and which often increases the ultimate conversion efficiency is called seeding. The nonlinear optical process which is at the foundation of OPO function is three-wave mixing, whereby a pump photon is converted into a signal photon and an idler photon. The standard coupled-wave theory of three-wave mixing shows that the rate of such conversion is proportional to the initial number of signal photons (or idler photons) within the nonlinear optical medium when the pump wave is initiated. In the absence of an external source of such photons, the only such photons are generated by vacuum fluctuations. As their density is very small, the process of amplifying these "intrinsic" signal photons into the desired signal is a difficult process.
The buildup of the signal wave can be accomplished more effectively by "seeding" the OPO. This is done by sending a small wave at the desired signal (or idler) wavelength into the nonlinear optical medium of the OPO along the OPO axis to be used just prior to initiating the pump pulse. Even a very small amount of seeding power (microwatts) will provide orders of magnitude greater signal photon densities at the start of OPO operation, which can significantly reduce the pump energy threshold and increase the conversion efficiency.
Recent work has shown that a process which significantly limits the conversion efficiency of pulsed OPOs is backconversion, which is conversion of a signal photon and an idler photon to a photon having the energy and propagation direction of the pump wave. Such loss of signal photons diminishes the signal wave and reduces the optical gain of the medium. The conventional response in the art to this reduction in the optical conversion of the medium is to increase the thickness of the nonlinear optical medium, thereby restoring the total gain of the OPO to a usable level. This design approach, although yielding functional OPOs, severely limits the conversion efficiency (signal power/pump power). Typical values for conversion efficiency are in the neighborhood of 10-20%.
There exists a long-standing need in this field for OPO systems exhibiting conversion efficiencies much greater than 20%. In particular, applications for OPOs include medical diagnosis, medical treatment, and remote sensing. These applications require moderate amounts of laser power at wavelengths inaccessible to highly efficient laser sources, and thus are candidates for OPO utilization.
The present invention seeks to satisfy the aforementioned needs by introducing a new class of OPO in which backconversion is greatly reduced. The principal technique for such reduction is to limit the density of idler photons propagating in the nonlinear optical medium of the OPO. Various embodiments and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.